At the present time, where a transfer of a high torque is necessary between the shaft and the machine element mounted thereon, the standard practice is to provide a keyway in the shaft and on the confronting cylindrical surface of the machine element. The keyways are registered and the shaft and element are interlocked by the insertion of a key in the keyways to span between the shaft and the machine element to mount the element on the shaft against relative rotation. The key is anchored in the hub of the machine element by a radially-disposed set screw. The form of mounting may permit limited axial adjustment of the element on the shaft but forecloses any circumferential adjustment since the positions of the keyways in the shaft and the machine element determine the circumferential position of the element on the shaft. Apart from the limited axial adjustment provided by the conventional keyed connection between the element and the shaft, the machining operations necessary to provide the keyways constitute a substantial cost factor in the production of the elements. Furthermore, during operation of such a connection over a period of time while power is transmitted, often there occurs fretting corrosion between the keys and keyways and between the set screw and its receiver, which makes it difficult to disassemble the connection when necessary, and eventually limits the torque transmitted.
Other means of fastening machine elements to shafts have been used such as splines, flats, set screws, etc. All have the same basic disadvantages: costly machining, lack of adjustment, fretting corrosion, and difficulty in assembly and disassembly.
Recognizing these difficulties, mounting elements hae been made available commercially which permit the mounting of machine elements on shafts without the use of keys and keyways. In one such device, four separate elements are mounted on the shaft and these elements are deformed by machine screws positioned in spaced relation circumferentially about the shaft to effectively clamp the unit onto the shaft and onto the machine element by frictional engagement. In such units, the effectiveness of the device depends upon uniform application of the torque about the entire circumference of the shaft and it is therefore necessary to tighten the screws equally around the entire circumference of the shaft and this requires step-by-step tightening of the screws to avoid distortion caused by unbalanced tightening. Inasmuch as this unit depends upon deformation of the metallic elements to frictionally engage the shaft, the tightening operation generates large stresses which, in turn, require substantial tightening force. A further drawback of this type of unit is the difficulty in loosening the unit when it is desired to remove the machine element or to make an adjustment thereof. In one conventional unit of this type, all of the tightening screws must be released gradually in a predetermined order to permit the stress upon metallic elements to be relieved and then additional breaking-away screws are incorporated so that the screws may be tightened to forcefully disengage the unit from the shaft.
To avoid the use of screws to deform the members, other commercial units have provided overlapping tapered elements which are concentric with the shaft and may be axially displaced relative to one another so that the axial displacement of the tapered surfaces causes the inner and outer elements to expand radially as they slide upon each other to thereby frictionally interlock the bore of the machine element with the outside surface of the shaft. In one of such devices, the confronting tapered surfaces are specially treated to provide a low coefficient of friction between them. By forcing the two tapered elements together axial pressure is exerted on the tapered members, the frictional difference causing the unit to frictionally grip both the shaft and the machine element, respectfully, without undue slippage. To prevent circumferential slippage between the two tapered elements, a key is provided between the members. While the difference is coefficients of friction embodied in the unit purports to facilitate breaking away of the components to release the mounting, in practice break-away screws are used to separate the tapered elements and permit release of the frictional engagement of the unit with the shaft and the machine element.
Most commercial units are fabricated in several pieces which must be assembled when put into use and which fall apart when the mounting unit is released to separate the machine element from the shaft. The separation of the component parts of the unit enables loss of individual components.
Furthermore, in most cases, the shaft or the machine element, or both, must be machined to accommodate fasteners or other retainers to retain the mounting unit in place.